Western influence nevertheless led to many reforms in the 19th century and major concessions, most notably the loss of a large territory on the east side of the Mekong to the French and the step-by-step absorption by Britain of the Shan and Karen people areas and Malay Peninsula.As part of the concessions which the Chakri dynasty offered to the British Empire in return for their support, Siam ceded four predominantly ethnic-Malay southern provinces to the British Empire in the Anglo-Siamese Treaty of 1909.The Chinese called this region Xian, which the Portuguese converted into Siam." (Baker and Phongpaichit, A History of Thailand, 8) A further possibility is that Mon-speaking peoples migrating south called themselves 'syem' as do the autochthonous Mon-Khmer-speaking inhabitants of the Malay Peninsula. 1851–1868) reads SPPM (Somdet Phra Poramenthra Maha) Mongkut King of the Siamese, giving the name "Siam" official status until 24 June 1939 when it was changed to Thailand.Thailand was renamed to Siam from 1946 to 1948, after which it again reverted to Thailand.Following the decline and fall of the Khmer empire in the 13th–15th century, the Buddhist Tai kingdoms of Sukhothai, Lanna, and Lan Xang (now Laos) were on the rise.
Ayutthaya became one of the most vibrant trading centres in Asia.Ayutthaya's expansion centred along the Menam while in the northern valleys the Lanna Kingdom and other small Tai city-states ruled the area.In 1431, the Khmer abandoned Angkor after Ayutthaya forces invaded the city.According to George Cœdès, "The Thai first enter history of Farther India in the eleventh century with the mention of Syam slaves or prisoners of war in" Champa epigraphy, and "in the twelfth century, the bas-reliefs of Angkor Wat" where "a group of warriors" are described as Syam.Additionally, "the Mongols, after the seizure of Ta-li on January 7, 1253 and the pacification of Yunnan in 1257, did not look with disfavor on the creation of a series of Thai principalities at the expense of the old Indianized kingdoms." The Menam Basin was originally populated by the Mons, and the location of Dvaravati in the 7th century, followed by the Khmer Empire in the 11th.